Transition state region in the A-Band photodissociation of allyl iodide—A femtosecond extreme ultraviolet transient absorption study

Abstract

Femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy based on a high-harmonic generation source is used to study the 266 nm induced A-band photodissociation dynamics of allyl iodide (CH2 =CHCH2I). The photolysis of the C-I bond at this wavelength produces iodine atoms both in the ground ((2)P3/2, I) and spin-orbit excited ((2)P1/2, I*) states, with the latter as the predominant channel. Using XUV absorption at the iodine N4/5 edge (45-60 eV), the experiments constitute a direct probe of not only the long-lived atomic iodine reaction products but also the fleeting transition state region of the repulsive nIσ(∗) C-I excited states. Specifically, three distinct features are identified in the XUV transient absorption spectrum at 45.3 eV, 47.4 eV, and 48.4 eV (denoted transients A, B, and C, respectively), which arise from the repulsive valence-excited nσ(∗) states and project onto the high-lying core-excited states of the dissociating molecule via excitation of 4d(I) core electrons. Transients A and B originate from 4d(I) → n(I) core-to-valence transitions, whereas transient C is best assigned to a 4d(I) →σ(∗)(C-I) transition. The measured differential absorbance of these new features along with the I/I* branching ratios known from the literature is used to suggest a more definitive assignment, albeit provisional, of the transients to specific dissociative states within the A-band manifold. The transients are found to peak around 55 fs-65 fs and decay completely by 145 fs-185 fs, demonstrating the ability of XUV spectroscopy to map the evolution of reactants into products in real time. The similarity in the energies of transients A and B with analogous features observed in methyl iodide [Attar et al. J. Phys. Chem. Lett. 6, 5072, (2015)] together with the new observation of transient C in the present work provides a more complete picture of the valence electronic structure in the transition state region. The results provide a benchmark for theoretical calculations on the nature of core-excited states in halogenated hydrocarbons, especially in the transition state region along the C-I reaction coordinate.

Photoinduced Heterocyclic Ring Opening of Furfural: Distinct Open-Chain Product Identification by Ultrafast X-ray Transient Absorption Spectroscopy

“…[21][22][23] The approach here utilizes ultrafast core-level spectroscopy to observe photoinduced dynamics in gas-phase photochemical reactions. [24][25][26][27] Timeresolved X-ray absorption spectroscopy is able to directly probe the frontier molecular orbitals of photoproducts such as radicals, [28][29][30][31] of Argon, which sets the position for time zero and also the instrument response function (IRF) of the apparatus (90 fs, Figure S1). Further details may be found in the Supporting Information.…”

Ultrafast Intersystem Crossing in Acetylacetone via Femtosecond X-ray Transient Absorption at the Carbon K-Edge

“…The sensitivity of the core-to-valence pre-edge resonances to the nature of the valence-excited states in polyatomic molecules provides valuable electronic structure insights into the photochemical reaction pathways and transition states. 42,[46][47] We report the photoexcited dynamics of AcAc by following the evolution of the key core-to-valence resonance peaks in the x-ray absorption spectra at the carbon K-edge and comparing them with TDDFT-simulated x-ray spectra of the excited states. The results show that the non-adiabatic passage to the T 1 state in AcAc is ultrafast and occurs much more rapidly than previously thought.…”

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

“…Owing to the importance of alkyl iodides as a benchmark system, direct observation of the conical intersection gating the formation of photoproducts is a very appealing target for both experiment and theory 35 – 37 . Although a number of time-resolved experiments on the dynamics of alkyl iodide photodissociation in the A -band have been reported using femtosecond XUV transient absorption and Coulomb explosion imaging 35 , 36 , 38 , 39 , limitations in the temporal resolution of previously-reported experiments have precluded a direct observation of passage through the conical intersection.…”